This invention relates a process for producing an aligned carbon nanotube film comprising a large number of aligned carbon nanotubes. The process is advantageous for producing aligned carbon nanotube films that find use as electron emitters, battery electrodes, gas separation films, sensors and in energy storage.
The carbon nanotube was first discovered by Sumio Iijima in 1991 (Nature, 354, pp. 56-58 (1991)). It is a carbon material, generally in the form of a very long (1-100 μm) hollow tube with a diameter of 1-100 nm. A wide range of potential applications have been proposed for the carbon nanotube and they include the use as electron emitters, battery electrodes, gas separation membranes, sensors and in energy storage. When a multiple of carbon nanotubes are to be used in these applications, the tubes are preferably aligned in one direction so that their individual features are integrated and assembled into a system in an efficient and easy manner. It has been demonstrated that in terms of electron emission characteristics and physical properties such as strength, nanotubes with smaller outside diameters are advantageous.
Commonly employed methods of producing carbon nanotubes include arc discharge with graphite electrodes, laser sublimation of graphite, and vapor-phase decomposition of carbon compounds using suspended catalytic metal particles. However, the many carbon nanotubes produced by these methods lack alignation and are unsuitable for shaping into a bundle or film.
Aligned carbon nanotube films or bundles of aligned carbon nanotubes can be formed by one of two methods, aligning separately produced carbon nanotubes on a substrate (Japanese Laid-Open Publication No. 2001-130904) and producing carbon nanotubes directly on a substrate. The latter method provides ease in achieving orientation in one direction and is a more advantageous method. Techniques for producing carbon nanotubes on a substrate include: (1) forming a catalytic metal membrane on a substrate, etching the membrane and thermally decomposing hydrocarbon on the substrate (U.S. Pat. No. 6,350,488); (2) preparing an iron-containing mesoporous silica substrate by a sol-gel method, reducing it with hydrogen and thermally decomposing acetylene on the substrate (Nature, 394, pp. 631-632 (1998)); (3) irradiating a substrate with plasma or microwaves to form carbon nanotubes (WO 99/043613); (4) forming a thin film of silicon carbide single crystal on a silicon substrate by epitaxial growth, separating it from the substrate by etching and heating it at high temperature in an oxygen-containing atmosphere (WO 98/042620); (5) anodizing an aluminum plate, electrodepositing cobalt on the bottom of the oxide film to prepare a substrate, reducing the substrate with carbon monoxide and thermally decomposing acetylene (U.S. Pat. No. 6,129,901); (6) forming a catalytic metal layer on a surface of a substrate by vacuum vapor deposition and thermally decomposing hydrocarbon (Japanese Laid-Open Publication No. 2001-220674); (7) preparing fine catalyst particles by a reverse micelle method or the like, loading them on a substrate and thermally decomposing hydrocarbon (Japanese Patent Laid-Open No. 2001-62299).
However, these methods involve cumbersome steps for preparing the substrate on which to form aligned carbon nanotubes and it is in only limited areas that they can form aligned carbon nanotubes. Hence, it has been difficult to realize economical production of large-area aligned carbon nanotube films that are suitable for use in the above-mentioned applications. In addition, in all approaches except method (4), the carbon nanotubes in aligned films have outside diameters greater than 20 nm.
Under the circumstances, the present inventors conducted intensive studies in order to develop a simple method of preparing an aligned film from carbon nanotubes having outside diameters of 20 nm and less and found that when a carbon compound was decomposed on a substrate that was coated with an element having no catalytic activity by itself and which loaded a metallic element having catalytic activity or a compound thereof, a carbon nanotube film was formed on the substrate surface as it was aligned in a direction perpendicular to the substrate (Japanese Patent Application No. 2001-120357).